In high-speed optical transmission, a number of detecting systems are known according to the modulation systems.
A heterodyne detecting system based on a reference carrier is described in “Degradation of Bit-Error Rate in Coherent Optical Communications Due to Spectral Spread of the Transmitter and the Local Oscillator,” IEEE/USA J. Lightwave Technol., Vol. LT-2, pp. 1024-1033, December 1984, by K. Kikuchi, T. Okoshi, M. Nagamatsu, and N. Henmi. This system, similar to the present wireless communication, regenerates a carrier from a received signal to stabilize its frequency using an optical PLL (phase locked loop). This system does not require differential encoding process and it is possible to realize a transmission capacity and a receiving sensitivity level near to the theoretical limits. However, it is necessary to control a polarization of the reference carrier or received signal such that they are parallel each other. In addition, it is difficult to realize an optical PLL. Moreover, it is required to suppress the influence of a phase noise of the reference carrier.
A delayed detection system for a DQPSK (Differential Quadrature Phase Shift Keying) signal is described in “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) transmission Using GaAs/AlGaAs Integration,” Proc of OFC2002, FD6, March 2002 by R. A. Griffin, R. I. Johnston, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, M. J. Wale, J. Hughes, P. A. Jerram, and N. J. Parsons. The DQPSK transmission is a system to carry information using phase differentials between bits. A DQPSK signal is easily converted into an ASK (Amplitude Shift Keying) signal using interference between bits and then a data is demodulated from the ASK signal by an existing demodulator. This delayed detection system detects a signal by regarding one-bit-prior signal as a local oscillating light. Since there is no need to regenerate a carrier, this system is practical and easy to be realized. However, in multi-level modulation, a transmitting side is required to operate complicated preceding. Also, it is not easy to comply with a change of a modulation method.
Another system is also well known in which a carrier signal is superimposed in a band of a modulated signal as a pilot carrier and a regenerated carrier is extracted through a filter. In this system, a carrier is separated and extracted by a filter and therefore a complicated filter control system is required to realize a stable receiving performance.
In a wireless communication system, a system to perform quasi-synchronous detection using a pilot symbol is well known as a kind of fading compensation system (see U.S. Pat. No. 4,899,367). In this system, a local reference signal generator in a receiving station performs temporary detection and then the result is compensated by a pilot symbol. When this system is applied to optical transmission, it is necessary to make polarizations of a reference carrier and a received signal parallel with each other.
There is a transmission system to perform self-heterodyne detection by transmitting a pilot carrier in a frequency band other than the frequency band for data transmission. However, the pilot carrier needs to be sufficiently apart from the frequency band for data transmission and therefore the usability of the frequencies becomes inefficient.
A system to use a pilot signal and a regenerated clock signal is described by B. Wandernoth in “1064 nm, 565 Mbit/s PSK TRANSMISSION EXPERIMENT WITH HOMODYNE RECEIVER USING SNNCHRONISATION BITS,” ELECTRONICS LETTERS. Vol. 27, No. 19, September 1991. However, this system has a problem that transmission speed in an electric circuit is limited.
A synchronous detection system is described by Satoshi Tsukamoto, Dany-Sebastien Ly-Gagnon, Kazuhiro Katoh, and Kazuro Kikuchi in “Coherent Demodulation of 40-Gbit/s Polarization-Multiplexed QPSK Signals with 16-GHz Spacing after 200-km Transmission,” Proc. Of OFC2005, PDP29, March 2005. This system AD-converts a received data at a high speed and digital-signal-processes the converted digital data to correct distortion of phase fluctuations. However, this system is not suitable for high-speed transmission since the performance of the AD-conversion and digital signal processing is limited.